Generally, concrete is a brittle material with high compressive strength but low tensile strength. In the concrete industry, all concrete work is typically specified on the basis of the compressive strength. Any attempt to improve the crack strength (tensile strength) and toughness of the concrete almost always requires the introduction of reinforcing addition. For example, rebar (steel rods) is added, which provides structural integrity but does not eliminate cracking. Metal mesh has also been added to reduce cracking but it cannot be used effectively to reinforce concrete of complex geometry.
Plastic fibers have also been used to improve the tensile strength and toughness (resistance to cracking). However, the addition of synthetic polymer fibers almost always causes a reduction in the compressive strength. In addition, when plastic fibers are used, they tend to only improve either the tensile strength (strength before the first crack appears) or the toughness (resistance to cracking), but not both at the same time.
Also, the addition of polymer fibers almost always worsens the workability of the concrete mix, resulting in (i) a practical upper limit for fiber volume/loading (typically about 0.3 volume percent) and (ii) increased difficulties with transporting concrete slurries for use in applications requiring shotcreting (sprayed concrete). The addition of extra water to improve workability reduces the mechanical properties of cured concrete and increases the tendency to form cracks due to shrinkage upon curing. To alleviate these problems, superplasticizer has been added to reduce the water content, but these invariably increase the cost and the slump (i.e., flow) of the concrete after it is applied. As a result, suitable concrete formulations for shotcrete applications suffer from either poor mechanical properties, insufficient workability or greatly increased cost.
Accordingly, it would be desirable to provide a concrete formulation, for example, that improves one or more of the problems of the prior art, such as improving the tensile strength, toughness and workability, while not decreasing the compressive strength described above.